Hey!
If you go out to eat, try to stick to salad I guess. If you do end up at a kebab place at 2 in the morning then have a small cicken kebab with plenty of salad and no sauce, drink vodka and diet coke, or gin & tonic, stay away rom the beer!
If your into a curry then have a veggie tomato based one with boiled rice, stay away from the nam bread!
There is plenty to see, so try to walk the city, view the art galleries, that will help keep down the excess levels too!
Also Manchester has a great nightlife of gigs and clubbing, so how about dancing off some weight as well & of course there is always to option of staying in and making sweet loooooovvvvvvveeeee!